This invention relates to data reading devices, and more particularly to a data reading device with a servo system adapted to perform the position control of a pickup data detection point.
In data reproduction devices, particularly in optical data reading devices, a so-called "tracking servo device" is employed in order to allow the data detection point of a pickup to accurately follow a record track on a recording surface. Furthermore, in order to correct the time axis of reproduction data, it is necessary to control the movement of the data detection point in the record track's tangential direction. For this purpose, a so-called "tangential servo device" is used.
The former tracking servo device will be described with reference to an optical data reading device. FIG. 1 is a block diagram showing the tracking servo device. In FIG. 1, reference numeral 1 designates a part of one record track on a recording surface; and 2, 3, and 4 designate light receiving elements for photoelectric conversion. In the optical data reading device, a spot of light is formed on the recording surface as the data detection point, and detects the optical data from the reflected light beam. More specifically, the light receiving element 2 is to receive a data detecting light beam (not shown) reflected from the recording surface, and the light receiving elements 3 and 4 are to receive tracking error generating light beams (not shown) reflected from the recording surface. When the data detecting light beam is on the center line of the record track 1, its reflected light beam is aligned with the center of the light receiving element 2, while the tracking error generating light beams are positioned on both side lines of the track and their reflected light beams are aligned with the centers of the light receiving elements 3 and 4, respectively.
Therefore, the shift of the data detecting point in a direction perpendicular to the record track 1 can be determined by detecting the difference between the outputs of the light receiving elements 3 and 4 with a subtractor 5. The output of the subtractor 5, which represents the amount of shift, is applied through an equalizer amplifier 6 and a loop switch 7 to a subtractor 8. The output of the subtractor 8 is amplified by a drive amplifier 9, as a result of which the latter produces a drive signal which is applied to a drive coil 11 adapted to control the deflection angle of an actuator, namely, a tracking mirror 10. The data detecting point can be moved in a direction perpendicular to the track by controlling the deflection of the tracking mirror 10. Current flowing in the coil 11 is detected by a resistor and inputted into the subtractor, thereby forming a feedback loop to stabilize the system.
On the other hand, the drive signal applied to the drive coil 11 is received be an LPF (low-pass filter) 13. The output (A) of the LPF is subjected to level decision by a window comparator 14. When the LPF output (A) exceeds a predetermined level range, the comparator 14 provides a detection output. The generation of the detection output triggers an MMV (monostable multivibrator) 15 which provides a single output (B) having a predetermined time width (T.sub.0) For the duration (T.sub.0) of the single output, the loop switch 7 is open and the tracking servo loop is open.
In the arrangement in FIG. 1, the loop switch 7 is ordinarily turned on during reproduction. Therefore, the tracking servo loop operates normally, and the data detecting point accurately follows the record track at all times. In order to retrieve record data, the pickup is sometimes quickly moved radially of the recording disk; i.e., a so-called scanning operation is carried out. In this case, the pickup is quickly moved with the tracking servo loop closed (on). However, since the angle of deflection of the tracking mirror 10 from the neutral point for moving the data detecting point is limited, it is necessary to return the tracking mirror 10 to the neutral point before the limited angle is reached.
For this purpose, the DC component of the drive signal of the coil 11 is detected by the LPF 13, the MMV 15 is triggered when the DC component reaches the upper limit value V.sub.H of the window comparator 14 as shown in FIG. 2A, and the servo loop is opened only for the duration T.sub.0 of the single pulse (FIG. 2B) outputted by the MMV, so that the mirror 10 is naturally returned to the neutral point. The period of time (T.sub.0) which is long enough for the mirror to return to the neutral point is set by the MMV 15. After this period of time, the servo loop is closed again, so that the scanning operation is carried out while the tracking operation is being effected.
In the above-described method, the drive signal to the coil 11 is zeroed when the mirror is returned to the neutral point. Therefore, for instance in the case where the quality factor Q of the actuator including the mirror is high at its resonance frequency, the motion of returning the mirror to the neutral point is a damping and vibrating motion, and accordingly it takes a relatively long period of time until the mirror stops completely.
This phenomenon occurs not only in the tracking servo system but also in the tangential servo system. That is, if the DC component of the time axis error of the reproduction signal is superposed during the scanning operation, it is necessary to open the tangential servo loop before the angle of deflection of the tangential mirror reaches the limit value, to thereby return the mirror to the neutral point. In this case also, the mirror is naturally returned merely by turning off the servo loop switch. Therefore, it takes a long period of time until the mirror returns to the neutral point.
The same difficulty occurs not only with the optical data reproducing device but also with other devices such as for instance an electrostatic type data reproducing device.